As suggested before, a Power rack would be your first choice. Could always sell those linear bearings and that steel stock and use that money towards a PR. Smith machines are not all that safe as compared to a power rack, giving a false sense of security. Also a Smith machines does not give the same strength curve to the natural muscle action. The BP for example has a "S" curve (three phased ..as most movements do) with-in the full ROM. A Smith machine may tend to put too much pressure where it is not desired, because of that fixed pattern you must follow when doing each rep. This happens mostly when using heavier weights, with injury to follow.
Many men use and like the Smith, making good progress on it. Usually moderate weight is used. So actually it's really up to you which suits your needs. But a better functioning piece of equipment, if heavy weights and safety are ever the option, would be a PR. Might get a short membership in a local gym, where there is a Smith machine, and try one out for yourself.
You could make a good PR with that steel you have, rather than buying one. You seem to know shop equipment. And probably ARC/TIG welding, cutting and pre fabbing mental. That's most of the battle anyway. Good Luck.